The present invention relates to an interference cancellation method in a data transmission system which utilizes a multiple access interference cancellation method in which estimates of interfering signals are subtracted from the desired signal, and in which method a confidence coefficient is calculated for the estimates obtained from the received signal.
In the design and implementation of data transmission methods, an essential problem is simultaneous transmission and reception of signals of several simultaneous users, so that the signals cause as little interference to each other as possible. Owing to this fact and the available transmission capacity, several different transmission protocols and multiple access methods have been developed, the most common of which in particular in mobile communication are the FDMA and the TDMA methods, and lately also the CDMA method.
The CDMA (Code Division Multiple Access) system is a multiple access method which is based on spread spectrum technology and whose application in cellular communication systems has lately been initiated along with the earlier FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) technologies. The CDMA technology has several advantages over the earlier methods, such as simplicity of frequency planning and spectral efficiency.
In a CDMA method, the narrow-band data signal of the user is multiplied by a spreading code of much wider bandwidth to a relatively wide traffic channel band. In the known experimental cellular network systems, the bandwidths used on traffic channels include, for example, 1,25 MHz, 10 MHz and 25 MHz. In the multiplying process, the data signal spreads to the whole band used. All users transmit simultaneously by using the same frequency band. A separate spreading code is employed for each connection between a base station and a subscriber terminal equipment, and the signals from the users can be identified from each other in the receivers on the basis of the spreading code of each connection. An attempt is made for choosing the spreading codes so that they are mutually orthogonal, i.e. they do not correlate with each other.
Correlators or adapted filters in CDMA receivers implemented in a conventional way are synchronized with the desired signal, which is identified on the basis of the spreading code. The data signal is returned in the receiver onto the original band by multiplying it by the same spreading code as in the transmission phase. The signals which have been multiplied by some other spreading code neither correlate nor return to the narrow band in an ideal case. They thus appear as noise from the point of view of the desired signal. The aim is thus to detect the signal of the desired user among several interfering signals. In practice, the spreading codes are not non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. This interference caused by the users for each other is termed as multiple access interference.
A data transmission method employing TDMA multiple access system has several frequencies in use, each of which is divided into time slots in which the signals of the various users have been placed. Thus, each user has a time slot of his own. As the frequency range reserved for the system is usually limited, the same frequencies must be used in cells located a certain distance away. If high frequency efficiency is desired, this distance should be kept as small as possible. This results in different transmissions on the same frequencies interfering with each other. Consequently, an interfering signal is heard in the receiver in a certain time slot in addition to the desired signal, which interfering signal originates in some other connection using the same frequency.
The single user detection method described above in connection with CDMA is not optimum because, in connection with the detection, it disregards information contained in the signals of other users. In addition, the conventional detection is unable to correct errors caused partly by unorthogonal spreading codes and signal distortion on the radio path. An optimum receiver takes into consideration the information contained in the signals of all the users, and thus the signals can be detected in an optimum manner by using, for example, a Viterbi algorithm. In the CDMA system, for example, an advantage of this detection method is that the situation, as far as the receiver is concerned, resembles a single user CDMA system in which the multiple access problem does not exist. For example, the near-far problem, typical for CDMA systems, does not occur. The term near-far problem refers to a situation where a transmitter close to the receiver covers the transmitters further away by its transmission. The most serious weakness of the Viterbi algorithm is that the computational capacity it requires increases exponentially as the number of users increases. For example, a system of ten users in which the bit rate is 100 kbit/s by QPSK modulation would require 105 million arithmetical operations per second for calculating the Viterbi algorithm. In practice, this constitutes a bar to the implementation of an optimum receiver.
However, it is possible to approximate an optimum receiver by various methods. The prior art knows different kinds of methods for multiuser detection (MUD). The best-known methods include a linear multiuser detection, a decorrelating detector and a multistage detector. These methods are examined in closer detail in the references Varanasi, Aazhang: Multistage detection for asynchronous code division multiple access communications, IEEE Transactions on communications, vol. 38, pp. 509-519, April 1990, Lupas, Verdu: Linear multiuser detectors for synchronous code-division multiple access channels, IEEE transactions on Information Theory, vol 35, no. 1, pp 123-136, January 1989, and Lupas, Verdu: Near-far resistance of multiuser detectors in asynchronous channels, IEEE Transactions on communications, vol 38, April 1990. However, these methods are also associated with operations, such as matrix inversion operations, requiring a lot of computational capacity.
A second way for solving the problems caused by the multiple access interference is to use interference cancellation (IC) methods. In IC type solutions, the purpose is to detect the users one by one, often in order of magnitude, so that the influence of the signals of users already detected is eliminated from the received signal prior to detection of the subsequent user. As an example of such a solution, reference is made to the European patent publication 491668, applying the method described above in the CDMA cellular mobile communication system. The interference cancellation methods are computationally more efficient than the algorithms of the MUD type, but their performance is weaker particularly during poor reception conditions, such as a fading multipath channel, often having low signal levels.
Multiple access interference cancellation methods similar to the ones described above can also be applied to TDMA systems. They do, however, have the deficiency of a deteriorating performance in case the interfering signals have bad estimates. In the worst case, multiple access interference cancellation may even increase interference, if the interfering signals are subtracted on the basis of wrong estimates.